For many years, attempts have been made to control rotary-type (boring-type) continuous miners through the use of gamma detectors and various other sensors. For those formations which contain characteristic radiation profiles, such as potash formations, gamma detectors have shown promise for controlling or assisting operators to control cutting. However, the results have generally been less than desired due to deficiencies in the gamma detectors, lack of system capabilities, and the manner in which the equipment has been employed.
There is a large economic value from being able to extract higher grade minerals while leaving the lower grade minerals in the mine. Not only is there a cost associated with cutting and removal of unwanted materials, but the good materials must be separated from waste materials and the waste materials must be disposed. There is also a need to calculate the grade of the ore being mined. Previous attempts at measuring the ore grade with gamma detectors have not been effective for some of the same reasons that the control of the cutting has not been effective. To measure the grade requires being able to actively measure the thickness of the beds, the radiation level from each bed, and to calculate the amount of material being mined from each bed. Previous and currently employed approaches do not provide sufficient precision for making accurate calculations, for reasons to be explained later.
Study of conventional attempts to control boring-type continuous miner cutting processes by use of gamma detectors has shown that when gamma detectors have been employed they have not been positioned properly in order to be able to make the needed measurements. Often, the detectors are located on the frame of the miner, 6-12 feet from the face of the formation being cut. Detectors located many feet from the face are measuring where the cutter has already passed instead of where the cutter is going. If a decision is made to move the cutter heads up or down, by the time that the gamma detectors can measure the effect of the decision, the mining machine, or crawler, will have climbed upon or dropped onto the newly cut floor that results from the cutting decision. In so doing, the effects of the cutting decision will be amplified and conditions are then set for an even more exaggerated response. Further, certain operational dynamics, if not corrected, will result in minor deviations from the ideal path, even if the formation is relatively constant. In addition, the detectors, being on the miner body, are located in fixed positions. Numerous fixed-position detectors, having specially selected fields of view, would be necessary to acquire precise radiation data from the beds to be observed. Physical limitations prevent using the needed number of detectors and make it essentially impossible to achieve the ideal fields of view.
There are thought to be many practical reasons why gamma detectors have not been placed near or on the rotating cutters of continuous miners. First is the difficulty of mounting a detector on the rotors, given the obvious space limitations and geometry. Either two or four rotors are used and these are interleaved as they rotate. If detectors are made small enough to fit in the available space, without interference with the opposing rotor, they may not be sufficiently sensitive. Another obstacle has been that most industrial gamma detectors cannot withstand the shock, vibration and abrasion associated with being near or on the cutter. Even if not broken by the environment, they are prone to produce false counts when subjected to shock or high vibration. These false counts degrade performance. Neither is it a simple matter to properly shield gamma detectors, while at the same time, maintaining a sufficiently large field of view, have good spatial resolution where needed, and be small enough to be properly positioned on a rotor.
Motion sensors or other suitable sensors, such as position sensors, and associated micro-controllers or processors are needed to correlate measurements with the position in the formation being mined and to process measurements from multiple beds of ore. Providing sufficient power to the sensors and transmitters on the rotor is another special need that must be addressed when locating the equipment on the rotor. Providing power to detectors on the rotors requires either battery modules or gravity-driven generators on the rotors, brushes (slip rings) to transfer power across the rotating shaft that supports the rotors or some other suitable mechanism. Transmitting data from the detectors to the control systems on the miner body is another challenge. Collectively, having to make these and other special provisions, in addition to the fundamental limitations of available detectors, have discouraged using a rotor-mounted gamma detector system or have resulted in disappointing failures.
The need for an effective boring-type continuous mining system remains. Rotor-mounted gamma detectors and associated hardware can produce a more effective control system but present many new problems to be solved.
U.S. Pat. No. 6,435,619, for example, the entire contents of which is hereby incorporated herein by reference, includes certain innovations generally directed to room and pillar mining or long wall mining using a drum-type continuous mining machine.
Although aspects of the present invention are amenable for use in the mining of numerous types of ores, the present invention will be described primarily in reference to the mining of potash, but it can apply to other types of mining where radiation exists in the formation. Underground mining of coal and trona are examples.
Vertical control of a continuous miner in a potash formation requires knowledge about the relationship of the miner to the mineral bearing beds in the formation as well as knowledge about the quality of the ore in those beds. Radiation from potash is generally proportional to the concentration of potash in the mineral bed. When there is a greater concentration of potash, the radiation level is also greater. By measuring the amount of radiation emanating from the mineral beds through which the miner is cutting, the position of the miner relative to these beds can be determined. Data from such measurements can also be used to determine the ore grade being mined.
Since the thickness of mineral beds vary, determining the position of the miner relative to a single bed is not a sufficient basis for good control. Accurate information about the thickness of the beds is also required. Or, said another way, it is necessary to know the position of multiple bed boundaries through which the miner is cutting. It is also important to know the quality of the ore in the beds at the top (also referred to as the “roof” or “back”) and the bottom (also called the “floor”) of the tunnel. Knowledge of position and ore grade allows systematic steering of the miner through the geologic formation so as to optimize the quality of the mineral that is mined.
Roll control is also needed to keep the miner aligned with the floor, which is often tilted from side to side. To accomplish roll control, i.e., tilt from side to side, knowledge of the relationship of the miner to the mineral beds on both sides of the miner is required. If gamma measurements are used as the basis for roll control, it is necessary to acquire gamma data from the walls on each side of the miner so that the cutter heads can be positioned to correspond with the tilt of the bed. Having done so, the miner will advance upon the newly cut floor, having a different tilt, side to side, and thus become realigned to the bed. As the tilt of the miner changes to match that of the beds being mined, the rotary cutter heads will return to their nominal orientation relative to the miner body. To achieve good control, the cutter heads must be made to respond to information about the formation at the cutter, ahead of the miner.
Since the mining equipment, including any gamma detectors being used, is radiated by all of the exposed surfaces of the formation being mined, sensors must be shielded from directions other than the target area for the bed of interest. In other words, the sensor must look for radiation coming only from the direction of the desired target area and not see the radiation from all other directions.
Minerals, such as potash, exist in layers or beds. Measurements made edgewise into the plane of a bed will produce data that is more easily interpreted than measurements made perpendicular to a bed. When looking perpendicular to the beds, it is difficult to determine which portion of the radiation is originating from the bed nearest the detector and which is emanating from beds farther away. This can be overcome to some degree by using spectral gamma data, rather than only gross gamma counts.
An ideal arrangement, if it could be achieved, would be to install a small detector near the tips of one arm on each rotor so the detectors could measure the radiation arriving in-plane from the beds in the face being cut. With this configuration, the detectors would exclusively be looking forward of the cutter. If it were achievable, having detectors positioned in this way should allow good vertical control. However, there are practical problems with implementing such an ideal arrangement. It would be very difficult, if not impractical, to attach sensors so that they could be extended and retracted along with the cutters on the rotors. Extendable cutters are necessary to avoid the cutters from getting wedged between the roof and the floor due to natural sagging of the roof as the tunnel is cut. Also, in order to provide a small field of view (FOV), encompassing a small solid angle regardless of the position of the rotor, a collimation device of considerable length is required, unless the scintillation element is made small. Adding a large collimation tube makes the job of mounting near the tips even more difficult. But, if the scintillation element is made small so that the shielded collimation tube is small, the count rate is too low to provide statistically accurate data. Further, if the FOV is made small compared to the scintillation element, the ratio of solid angle encompassing the target area in the formation is very small compared to the solid angle of all other directions. Then, a large amount of shielding becomes necessary to reduce background counts. Collectively, these considerations make it impractical to position a gamma detector near the tips, looking into the face.
Faced with the above problem, one might choose to place a detector further down on the rotor arm, nearer the hub. This is not satisfactory because the collimation requirement increases as well as shielding for the collimator, and neither space nor position allows typical detectors to be positioned such that they will be effective. The problem is perceived to be too difficult to overcome, particularly if conventional industrial gamma detectors are used. To overcome the above problems, a special combination of hardware design and system strategy is required.
There is a method and system architecture that will produce excellent results. A detector can be positioned on a rotor arm such that it is viewing in the radial direction into the surface of the circular path being cut out by the rotary cutter. If the detector has a low profile and does not extend very far down the rotor arm toward the hub, it will fit nicely and will be close to the mineral surface. This location is ideal for using a detector having a relatively large length to diameter ratio, due to a number of important reasons. First, it is important to provide a narrow field of view in the direction perpendicular to the beds but equally important to have a viewing area large enough to provide adequate sensitivity. A scintillation element size that is well suited for this application is 2 inches by 10 inches and should be oriented so that its long axis is parallel with the mineral beds. Having this shape, it is easier to configure the shielding to produce a FOV that is narrow in one direction, the direction in which good spatial resolution is needed, and wide in the other direction where spatial resolution is much less important. When the rotor arm is horizontal and the detector is looking horizontally into the beds, edge-wise, the narrow field of view is important to the rejection of radiation from beds above and below the one being measured. When the arm is vertical, good resolution is desired to detect the crossing of the interface at the roof. Having a wide FOV in the front-to-back direction, parallel to the mineral beds, increases sensitivity without reducing the resolution is the tangential direction for all rotor positions.
Another important advantage of using a scintillation element having an elongated shape is that it does not require much space above the rotor arm. This is particularly important for those rotor designs that have little space between opposing rotors. Also, the shape allows the entire detector to be placed near the end of the rotor arm so that it is beyond the path of the end of the arm on the opposing rotor, except for a section that can be low enough to not collide with the arm on the opposing rotor. If a conventional crystal configuration is used, having a round surface exposed to the radiation, a round collimation tube is required. This length of the tube must be at least twice the diameter of the crystal, and probably considerably longer in order to achieve acceptable spatial resolution. Instead, a detector, as proposed here, that has a relatively small diameter provides good spatial resolution and also makes room for adding more shielding to reduce background radiation thus improving the signal to noise ratio.
Some explanation may be helpful in better understanding the effect of detector FOV on sensitivity. The total sensitivity for a detector of a given size and type is the integration of the viewing area over the area of the crystal. It is more correct to say that the sensitivity is the integration of the volume of mineral emitting radiation to the detector over the volume of the detector. Since gamma rays have a wide range of energies, particularly after being scattered during their journey from their source to the detector, some will be stopped and counted near the surface of the scintillation element. The higher energy rays will tend to penetrate further into the scintillation element before being detected. Some will actually pass through the scintillation element and not be counted. Therefore, the total sensitivity of a detector is determined both by its exposed surface area and by its effective volume. For estimating and discussion purposes, it is sufficiently accurate to compare configurations based on areas rather than volumes so long as the average thicknesses of the scintillation elements being compared are reasonably similar.
This greater sensitivity is critical for a rotor-mounted detector because the view is constantly moving as the rotor turns. Data from each segment of the tunnel wall, roof and floor must be collected and summed over time in order to acquire enough data to overcome the statistical nature of radiation. Less sensitive detectors require more time to acquire the necessary data set.
In order to better understand the significance of the shape factor for a detector, a simplified discussion of the relationship of the detector shape and the desired FOV shape shows the importance of the factors. Consider a typical industrial gamma detector having a scintillation element that is 4 inches in diameter by 2 inches thick. On many miner rotor designs, there is not room for mounting a properly shielded detector that contains a 4 inch diameter crystal. But, assuming that there is room for a detector of that size, starting with the 4 inch diameter crystal and adding the materials necessary to support and protect it from the vibration environment and then adding thick shielding over the detector and over the collimation tube, results in additional height of at least 3 to 4 inches. Further, the collimation tube must be adequately long to sufficiently limit the FOV in the tangential direction, making the detector at least 8 inches high, and probably more. It is reasonable to assume that the collimation tube length will be at least twice the diameter of the scintillation element. This forces the center of the scintillation element to be approximately 20 inches away from the surface of the mineral. At 20 inches the main FOV, the area on the surface of the mineral that is in view of all the front surface of the scintillation element is approximately 20 square inches, or a circle that is approximately 5 inches in diameter. More importantly, the partial FOV that sees at least 50% of the volume of the scintillation element is approximately 75 square inches and is a circle approximately 10 inches in diameter. The cross-section surface area of such a 4 inch diameter crystal would be approximately 12.6 square inches. The sensitivity of the partial FOV will relate approximately to the product of 75 square inch partial FOV and the 12.6 square inch area.
In comparison, a properly designed 2 inch by 10 inch detector has a surface area of 20 square inches. The volume of the long crystal is also 25% greater than a 4 inch diameter by 2 inch thick crystal. Yet, it can be placed within about 10-11 inches from the mineral surface. Its height will be only about 65% that of the industrial 4 inch detector. The partial 50% FOV of this long detector in the tangential direction can be easily constrained to be only 9 inches, or less if desired, while its 50% partial FOV in the longitudinal direction, parallel to the plane of the bed, may be as much as 40 inches. A 9 inch by 40 inch FOV provides 360 square inches of viewing area which can be integrated over a crystal surface of 20 square inches. The partial FOV for this crystal will also be roughly related to the product of 360 and 20. After allowing for the distance factors, it is clear that the elongated crystal is much more sensitive. Given this greatly improved sensitivity, as compared with the 4 inch diameter scintillation element, the FOV of the 2 inch by 10 inch scintillation element can be reduced further as needed in the longitudinal direction and still maintain adequate sensitivity. But, in contrast, to reduce the field of the partial view of the 4 inch detector, the length of the collimation tube must be increased, when its-length would already a major problem for at least some rotor designs. In so doing, its already low sensitivity would be further reduced.
A gamma detector must have a light sensing device, such as a photomultiplier tube (PMT), to convert light flashes, produced by the scintillation element, into electrical signals. This PMT further increases the length of the gamma detector when length is already a serious problem. One option that might be considered would be to place the PMT inside the collimation tube, between the scintillation element and the source of the radiation. Although this is feasible, it is obviously undesirable because it blocks incoming radiation, particularly the lower energy radiation. These considerations again force one to consider using a smaller crystal, producing an overall smaller detector, and causing a serious loss of detector sensitivity. Again, there is an advantage to using the crystal shape proposed here because the smaller diameter makes it possible to use a smaller PMT, having a smaller face for light to enter. This is further improved upon by placing the PMT inside the hermetically sealed housing, with the scintillation crystal, so that window between and one optical coupler can be eliminated. Other special detector design features allow further reducing the length of the detector.
As can be understood from the foregoing discussions, there is a strong competition between the space available, the field of view needed, collimation requirements, a sufficiently large scintillation element, and shielding requirements. A typical industrial detector, having a circular scintillation face surrounded by a collimation tube, is not well suited to satisfy the configuration, performance, and reliability requirements. The above discussions were based on a hypothetical 4 inch diameter scintillation element in order to make a more understandable comparison. In reality, most, if not all, of the industrial detectors currently being used on boring-type continuous miners, are in fixed positions on the miner body, and have scintillation elements that are only 2 or 3 inches in diameter. Although these sizes are easier to integrate onto a rotor, their sensitivity is a serious limitation.
Since a detector on a rotor is moving rapidly, typically about 90° per second, the amount of gamma radiation that can be measured on a single revolution is not sufficient for control of the miner. Even for a highly sensitive detector configuration and location as already described, it remains necessary to accumulate gamma data over time for each segment of interest. A miner of this type is typically advancing slowly, between 1 and 2 feet per minute, and changes in the tilt of the formation occur slowly, over many feet of travel, the readings from many revolutions of the rotor can be combined. Also, undesirable excursions by the miner due to operational dynamics are usually small in a 1 to 2 minute period. Therefore, it is feasible to combine gamma data for multiple revolutions. To combine the data from many revolutions requires a method to correlate each gamma count with a position and to sum the measurements made for each position. This will be described in more detail later.
Some scintillation materials, such as sodium iodide, are very fragile and easily damaged. Gamma scintillation elements are also known to produce false counts under vibration and shock, requiring special packaging to be made noise-free. In order to fabricate a scintillation element, particularly one having a relatively large length to diameter ratio, such as a 2 inch diameter by a 10 inch length, special packaging is required for multiple reasons. This will be described in detail later. In order to conserve space, the size of the detector must be made as small as possible while still providing a suitably sized scintillation element. Provisions have been made in this invention to make the detector as compact as possible as will be described in detail later.
A well-designed gamma detector for this application provides for the photomultiplier tubes (PMTs), or other light detecting devices, to be directly coupled to the scintillation crystal with an oil-ringed coupler or some other reliable, high efficiency coupling. Elimination of the window that is normally between the PMT and crystal helps to achieve the highest possible performance. The design must be a very rugged, compact design which is crucial to achieving the needed performance within the limited space available. It must withstand the forces from mining, shock, vibration, and abrasion. Support of the critical PMT/scintillation element within the detector can be accomplished by metallic supports (such as flexible sleeves). Similar flexible sleeves, of a special dual-type, support the electronics and motion sensors in the armor. These devices provide excellent dynamic properties and use a minimum of space. The flexible sleeve for the scintillation element has gaps to minimize attenuation of incoming gamma radiation. The light sensing element, such as a PMT, must be a rugged type. This armor should be lead-filled, or be fabricated using a strong, high density metal such as tungsten, in order to reduce background radiation. A non-metallic high strength window is used in the armor to protect the scintillation element and other hardware from the ore being mined while providing little attenuation of the incoming gamma rays.
Rate gyros are used to determine the direction that the detectors are pointing so that gamma counts can be correlated with position. An accelerometer is used to remove long term drift, or offset, from the gyro outputs and to relate the positions of the rotors relative to gravity. An alternate method for removing long term drift from the rate gyro measurement may include magnetic switches or mechanical switches, proximity switches, laser beams, shaft position indicators, acoustic sensors, etc. For example, a magnet can be placed on the opposing rotor such that it will trip a magnetic sensor in the detector assembly whenever the opposing rotor approaches the rotor on which the detector assembly is located. It is possible to place such a magnet at other locations on the miner. Use of the accelerometer is preferred technically because it is an independent measure relative to gravity, can be contained entirely within the detector assembly, and does not require any external support.
Each detector contains the electronics, within the control module, required to acquire and transmit data and/or decisions to the miner and to each other. Typically, these electronics include at least three micro-processors, one for gamma, motion and logic control. The logic control section acquires data from the other two, makes logical decisions, and then sends the decisions and data to the miner as needed. If one detector is a master and the other a slave, most of the decision making is in the master. PIC microcontrollers are preferred to perform these processing functions because of their small size and low power requirements.
Passing data from one detector, sometimes called a Slave Detector, to another detector, called a Master Detector, can be accomplished by different techniques. RF transmitters and receivers are available in very small packages and can be integrated into the detectors for this purpose. Also, the data can be sent from one detector through brushes, slip rings, or encoder assemblies to the miner frame and then be sent through a second set of brushes, slip rings, or encoder assemblies to the detector on the other rotor. Information can be transmitted in both directions, using such techniques, as needed. Signal drivers are included in the control module to ensure adequate signal-to-noise ratio for that data transmitted by any of these techniques. Also, data and/or cutting decisions from the detectors may be sent to the miner in a similar manner.
In addition to sending cutting decisions to the miner control hydraulics, data may be transmitted for display to the operator. The control module will divide the full rotation of the rotor into many segments around the circumference. A typical segment would be 4 inches along the circumference. Gamma radiation is recorded continuously and gamma counts are correlated with the segment in its field of view. If a detector is added to the second rotor for roll control purposes, the data from that detector will be automatically associated with the appropriate segment, which will be in a different position in the cycle of the second rotor. Once on each rotation, the radiation detector will send the data along with steering decisions, via an RF link, collected on the previous revolution of the rotor. If an RF link is used, in order to save power, the RF transmitter may only be turned on for a brief time on each revolution and at the time when the rotor is in the best position for transmission. If the miner is equipped with slip rings for each rotor shaft, that would be a preferred arrangement, eliminating the need for RF equipment.
A special power source may be required for the portions of the geosteering system on the rotor. This can be provided by a set of nickel cadmium batteries that are located in another armored box located near the detector. Examples of other means of providing power to the system are to utilize a gravity driven system, or provide a means of transferring power from the miner across the shaft to the rotor.